In recent years, premixed compression self-ignition (homogeneous charge compression ignition (HCCI)) type engines which operate with high efficiency and discharge low amounts of NOx have been drawing attention. Because premixed compression self-ignition (HCCI) combustion enables operation under a leaner mixture than spark ignition (SI) combustion, it has the advantages of increased thermal efficiency and decreased the maximum combustion temperature. However, the control of ignition timing is difficult, and despite of ignition timing control utilizing internal EGR and so on, the operational range ensuring stable combustion is still limited. Therefore, an engine switching between HCCI combustion and SI combustion in accordance with its operational range has been proposed. An example of such an engine is disclosed in Patent Document 1. Various conditions of HCCI combustion and SI combustion differ, such as the amount of EGR gas and that of the mixture required in the combustion chamber. For example, when switching from SI combustion to HCCI combustion, the  amount of gas in the combustion chamber which is composed of the mixture and EGR gas (referred to hereinafter as in-cylinder gas) needs to be increased. On the other hand, the operational range allowing HCCI combustion to be performed stably ranges from an intermediate-rotation intermediate-load side to a low-rotation low-load side. Accordingly, at the time of switching combustion, the throttle valve in the intake passage tends to close, and the pressure is negative in a space from a region downstream of the throttle valve to an intake port located short of each combustion chamber. Therefore, even if the throttle valve is controlled so as to fully open upon switching combustion types, the amount of the mixture supplied to the combustion chamber is insufficient, causing a torque step in the form of a drop in torque.
To solve this problem, Patent Document 1 proposes that upon switching from SI combustion to HCCI combustion in a premixed compression ignition type engine with a supercharger, switching to HCCI combustion is performed only after the conditions for performing HCCI combustion are fulfilled by raising the pressure and temperature in the combustion chamber using the supercharger.
Patent Document 1: JP 2004-176688 A